1. Field of the Invention
This invention relates to a drive control apparatus for a brushless DC motor mounted on a compressor or the like and a method for driving the brushless DC motor.
2. Description of the Related Art
A brushless DC motor includes a stator having three phase windings and a rotor having permanent magnets.
When the brushless DC motor is driven, a DC voltage circuit for outputting a DC voltage and a switching circuit receiving the output voltage of the DC voltage circuit are used.
The switching circuit has three series circuits each having a pair of switching elements series-connected on the upstream side and downstream side with respect to current flow. Each of the connection nodes between the paired switching elements of the series circuits is connected to a corresponding one of the phase windings of the brushless DC motor.
The respective phase windings of the brushless DC motor are sequentially energized by continuously setting one of the upstream side switching element of one of the series circuits of the switching circuit and the downstream side switching element of a different one of the series circuits in the ON state, intermittently turning ON the other switching element and sequentially changing the switching elements to be turned ON. Energization of the respective phase windings generates magnetic fields from the respective phase windings. Interaction between the thus generated magnetic fields and the magnetic fields created by the permanent magnets of the rotor rotates the rotor. Switching of the energization of the respective phase windings is called commutation.
When the rotor is rotated, a voltage is induced in a phase winding which is not energized. The induced voltage is derived out and the rotation position of the rotor is detected based on a variation in the induced voltage. The timing of energization switching (commutation) for each phase winding is controlled according to the detected rotation position. The energization switching is repeatedly effected to continuously rotate the rotor.
One example of the driving control for the brushless DC motor is disclosed in Japanese Patent Specification No. H.1-13318 and Japanese Patent Disclosure No. H.2-142383.
In the example in Japanese Patent Specification No. H.1-13318, an inverter (4) is driven to sequentially energize the respective phase windings of a brushless motor (5). Then, the rotation position of a rotor (6) is detected by use of position detectors (71, 72, 73). Energization of each phase winding is controlled in a period of 120 electrical degrees according to the detected rotation position. In the energization of the period of 120 electrical degrees, energization of a front half period of 60 electrical degrees is continuously effected and energization of a latter half period of 60 electrical degrees is intermittently effected (subjected to the pulse-width modulation).
In the example in Japanese Patent Disclosure No. H.2-142383, a semiconductor switching element (2) is driven to sequentially energize the respective phase windings of a brushless motor (1). Then, a voltage induced in each phase winding is sampled and fetched by a microcomputer (5) and processed in the microcomputer (5) so as to detect the rotation position of a rotor of the brushless motor (1). Energization of each phase winding is controlled according to the detected rotation position.
At the time of switching of energization of the phase windings of the brushless DC motor, a counter electromotive force is generated in a phase winding whose energization is to be interrupted. When the counter electromotive force is generated, a current will flow from the switching circuit to the DC voltage circuit. The reverse flow of current reduces the service life of the electrical parts of the DC voltage circuit and at the same time reduces the rotation torque of the brushless DC motor. If the rotation torque of the brushless DC motor is reduced, the operation efficiency of the brushless DC motor is lowered and large noises and vibration are generated from the brushless DC motor.